1. Field of the Invention
The present invention relates generally to magnetic recording media, and more particularly to magnetic recording disks formed on non-metallic substrates and methods for their production.
Thin film magnetic recording disks generally comprise a disk substrate having a magnetic layer and a number of underlayers and overlayers deposited thereon. The nature and composition of each layer is selected to provide desired magnetic recording characteristics as generally recognized in the industry. An exemplary present day thin film disk is illustrated in FIG. 1 and comprises a non-magnetic disk substrate 10, typically composed of an aluminum alloy. An amorphous nickel-phosphorous (Ni-P) underlayer 12 is formed over each surface of the disk substrate 10, typically by plating. The Ni-P layer is hard, and imparts rigidity to the aluminum alloy substrate. A second underlayer in the form of a chromium ground layer 14 is formed over the Ni-P layer 12, typically by sputtering, and a magnetic layer 16 is formed over the ground layer 14. The magnetic layer 16 comprises a thin film of a ferromagnetic material, such as a magnetic oxide or magnetic alloy. Usually, a protective layer 18, such as a carbon film, is formed over the magnetic layer 16, and a lubricating layer 20 is formed over the protective layer.
The amorphous Ni-P underlayer 12 effectively smoothes out the irregularities in the surface of an aluminum substrate. The presence of the Ni-P underlayer 12, together with the chromium ground layer 14, has been found to improve the recording characteristics of the magnetic layer 16. In particular, a chromium ground layer formed over a Ni-P layer provides enhanced coercivity and reduced noise characteristics. Such improvements are further enhanced when the Ni-P underlayer is treated by mechanical texturing to create a roughened surface prior to formation of the chromium ground layer. The texturing may be circumferential or crosswise, with the preferred geometry depending on the particular composition of the cobalt-containing magnetic layer.
The outer carbon protective layer 18 serves a very different purpose. This protective layer has been found to greatly extend the life of magnetic recording media by reducing disk wear. The use of carbon as a protective outer layer is known for both metallic and non-metallic substrates. Carbon has been shown to provide a high degree of wear protection with a thin protective layer.
Such magnetic recording disk constructions have been very successful and allow for high recording densities. As with all such successes, however, it is presently desired to provide magnetic recording disks having even higher recording densities. For reasons best explained elsewhere, recording densities can be improved by reducing the spacing between the recording transducer (read/write head) and the magnetic disk surface while the disk is rotating. Such close spacing, in turn, requires magnetic recording disks which have very flat surfaces and which are dimensionally stable so that the surfaces remain flat during use. To this end, the use of dimensionally stable non-metallic disk substrate materials has been proposed, such as glass, ceramics, glass-ceramic composites, carbon, carbon-ceramic composites, and the like.
Such non-metallic disk substrate materials, however, suffer from their own disadvantages. Although non-metallic substrates may have highly polished surfaces, they still benefit from such a layer to smooth out irregularities. Glass substrates, for example, must be chemically strengthened so that they can withstand manufacturing and use. One method used is to subject glass surfaces to potassium-sodium ion exchange process to strengthen the outer surfaces. Such strengthening causes irregularities in the glass surface which must be polished out. Any irregularities which are not smoothed out can compromise the improved mechanical characteristics of a non-metallic substrate.
For these reasons, it would be desirable to provide an improved magnetic recording disk having a seedlayer deposited on a non-metallic substrate which provided improved magnetic recording characteristics of a subsequent conventionally deposited magnetic layer. It would be particularly desirable if such a seedlayer provided the magnetic recording disks with the enhanced magnetic properties available for aluminum and other metallic disk substrates, as well as the improved mechanical characteristics derived from the non-metallic substrates. The seedlayer should smooth out the irregularities of a non-metallic substrate surface and should improve the recording characteristics of a subsequent magnetic layer when applied to a non-metallic substrate.
2. Description of the Background Art
International Patent Application No. WO 88/05953, describes the use of a protective carbon outer layer on a magnetic recording disk with an aluminum substrate. U.S. Pat. No. 5,153,044 also discloses an aluminum substrate disk having a protective carbon outer layer, and further discloses the use of a typical amorphous Ni-P underlayer with an aluminum substrate. A similar use of a protective carbon outer layer with a non-metallic substrate disk is suggested by U.S. Pat. No. 5,316,864. U.S. Pat. Nos. 5,135,808 and 5,268,217 disclose the application of a diamond-like carbon film directly to a glass substrate to provide an optically transparent wear resistant surface. Other examples of protective carbon outer layers for magnetic recording disks are found in U.S. Pat. Nos. 5,080,948; 5,087,481; and 5,134,038.
U.S. Pat. Nos. 4,735,840 and 5,316,864 disclose a magnetic recording media having a glass substrate and a sputtered chromium nucleating under layer below the magnetic layer. An apparatus and process for producing disks having such a chromium nucleating layer is disclosed in U.S. Pat. Nos. 4,894,133 and 5,082,747. U.S. Pat. No. 5,119,258 discloses a similar sputtered chromium underlayer on a metallic substrate. An exemplary adhesion layer of group IVB, VB, and VIB transition metals applied to non-metallic substrates is disclosed in co-pending U.S. patent application Ser. No. 08/001,379. U.S. Pat. No. 5,094,898 is of general interest.